Searching for code hosts that require SSH is slow

Created by: eseliger

Currently, this store method in batch changes requires to be able to read the external_service_repos.clone_url to determine whether a repo is cloned using SSH. Since that column may be encrypted in the future, a text search over it is not future-proof. Also, it is slow on Sourcegraph.com. Adding a separate column to the repo table that stores a boolean whether it uses SSH should help.